1. Field of the Invention
The present invention pertains to a particular (Ti or TiNx)/TiN/TiNx barrier/wetting layer structure which enables the warm aluminum filling of high aspect ratio vias while providing an aluminum fill exhibiting a high degree of  less than 111 greater than  crystal orientation aluminum.
2. Brief Description of the Background Art
Titanium nitride layers have been used in semiconductor device structures as barrier layers for preventing the interdiffusion of adjacent layers of materials such as aluminum and silicon, for example. However, the crystal orientation of aluminum deposited over the surface of the titanium nitride barrier layer is typically polycrystalline, and polycrystalline aluminum has poor electromigration resistance.
In the formation of integrated circuit interconnect structures, such as a Ti/TiN/TiNx stack, electromigration of aluminum atoms within the aluminum layer becomes a problem if the aluminum layer is not formed with a high degree of  less than 111 greater than  crystal orientation. Electromigration of the aluminum atoms can result in open circuits within the integrated circuit structure, and therefore, such electromigration must be inhibited or eliminated. Electromigration of aluminum atoms can occur within filled vias as well, impairing the conductivity of the contacts.
U.S. Pat. No. 4,944,961 to Lu et al., issued Jul. 31, 1990, describes a process for partially ionized beam deposition of metals or metal alloys on substrates, such as semiconductor wafers. Metal vaporized from a crucible is partially ionized at the crucible exit, and the ionized vapor is drawn to the substrate by an imposed bias. Control of substrate temperature is said to allow non-conformal coverage of stepped surfaces such as trenches or vias. When higher temperatures are used, stepped surfaces are planarized. The examples given are for aluminum deposition, where the non-conformal deposition is carried out with substrate temperatures ranging between about 150xc2x0 C. and about 200xc2x0 C., and the planarized deposition is carried out with substrate temperatures ranging between about 250xc2x0 C. and about 350xc2x0 C.
S. M. Rossnagel and J. Hopwood describe a technique of combining conventional magnetron sputtering with a high density, inductively coupled RF plasma in the region between the sputtering cathode and the substrate in their 1993 article titled xe2x80x9cMetal ion deposition from ionized magnetron sputtering dischargexe2x80x9d, published in the J. Vac. Sci. Technol. B. Vol. 12, No. 1, January/February 1994. One of the examples given is for titanium nitride film deposition using reactive sputtering, where a titanium cathode is used in combination with a plasma formed from a combination of argon and nitrogen gases.
U.S. Pat. No. 5,262,361 to Cho et al., issued Nov. 16, 1993 describes a method for forming single crystal aluminum films on the surface of a substrate such as silicon (111). The object is to increase the amount of the aluminum (111) crystal orientation, to improve the electromigration resistance of the aluminum. Electrically neutral aluminum is deposited by a vacuum evaporation technique upon a silicon wafer surface at a temperature ranging between about 300xc2x0 C. and about 400xc2x0 C.
U.S. Pat. No. 5,543,357 to Yamada et al., issued Aug. 6, 1996, describes a process for manufacturing a semiconductor device wherein a titanium film is used as an under film for an aluminum alloy film to prevent the device characteristics of the aluminum alloy film from deteriorating. The thickness of the titanium film is set to 10% or less of the aluminum alloy film containing no silicon, the titanium film is set to 5% or less of the aluminum alloy film. The aluminum film is formed at a substrate temperature of 200xc2x0 C. or less by a sputtering process, and when the aluminum film or an aluminum alloy film is to fill a via hole, the substrate is heated to fluidize the aluminum. The pressure during the aluminum film formation and during the fluidization is lower than 10xe2x88x927 Torr. A titanium nitride barrier layer may be applied on an interlayered insulating film (or over a titanium layer which has been applied to the insulating film), followed by formation of a titanium film over the titanium nitride film, and finally by formation of the aluminum film over the titanium film. After formation of the titanium nitride barrier layer, the barrier layer is heated to a temperature of about 600xc2x0 C. to 700xc2x0 in a nitrogen atmosphere using a halogen lamp so that any titanium which is not nitrided will become nitrated. The titanium nitride barrier layer is said to be a poor barrier layer if un-nitrided titanium is present within the layer.
U.S. Pat. No. 5,571,752 to Chen et al., issued Nov. 5, 1996, discloses a method for patterning a submicron semiconductor layer of an integrated circuit. In one embodiment, titanium or titanium nitride having a thickness of between approximately 300 and 2000 xc3x85 is formed by sputter deposition to reach the bottom of a contact opening. The barrier layer may be annealed to form a silicide in the bottom of the opening. A conformal conductive layer of a refractory metal or refractory metal silicide is formed over the titanium or titanium nitride using chemical vapor deposition (CVD). Finally, a second conductive layer, typically aluminum is applied over the surface of the conformal conductive layer. The aluminum is sputtered on, preferably at a temperature ranging between approximately 100xc2x0 C. and 400xc2x0 C. This method is said to make possible the filling of contact openings having smaller device geometry design requirements by avoiding the formation of fairly large grain sizes in the aluminum film.
U.S. patent application Ser. No. 08/511,825 of Xu et al., filed Aug. 7, 1995, assigned to the Assignee of the present invention, and hereby incorporated by reference in its entirety, describes a method of forming a titanium nitride-comprising barrier layer which acts as a carrier layer. The carrier layer enables the filling of apertures such as vias, holes or trenches of high aspect ratio and the planarization of a conductive film deposited over the carrier layer at reduced temperatures compared to prior art methods.
U.S. patent application Ser. No. 08/753,251 of Ngan et al., filed Nov. 21, 1996, describes a method for producing a titanium nitride-comprising barrier layer on the surface of a contact via. For certain contact geometries, when the reactor pressure is reduced during formation of the titanium nitride-comprising barrier layer, the thickness of the barrier layer on the sidewalls of the via increases. This enables an aluminum fill to travel along the sidewalls of the via more easily, resulting in a better fill of the via. In particular, the titanium nitride comprising barrier layer needs to be of a minimum thickness and to have a minimum titanium content so that the barrier layer can react slightly with the Aluminum, to draw the aluminum along the sidewalls of the via.
U.S. patent application Ser. No. 08/825,216 of Ngan et al., filed Mar. 27, 1997, discloses various process techniques which can be used to control the crystal orientation of a titanium nitride barrier layer as it is deposited. Further, by increasing the {200} crystal orientation of the titanium nitride barrier layer, the resistivity of this layer is decreased.
A xe2x80x9ctraditionally sputteredxe2x80x9d titanium nitride-comprising film or layer is deposited on a substrate by contacting a titanium target with a plasma created from an inert gas such as argon in combination with nitrogen gas. A portion of the titanium sputtered from the target reacts with nitrogen gas which has been activated by the plasma to produce titanium nitride, and the gas phase mixture contacts the substrate to form a layer on the substrate. Although such a traditionally sputtered titanium nitride-comprising layer can act as a wetting layer for hot aluminum fill of contact vias, good fill of the via generally is not achieved at substrate surface temperature of less than about 500xc2x0 C.
To provide for aluminum fill at a lower temperature, Xu et al. (as described in U.S. patent application Ser. No. 08/511,825), developed a technique for creating a titanium nitride-comprising barrier layer which can act as a smooth carrier layer, enabling aluminum to flow over the barrier layer surface at lower temperatures (at temperatures as low as about 350xc2x0 C., for example). A typical barrier layer described by Xu et al., is a combination of three layers including a first layer of titanium (Ti) deposited over the surface of the via; a second layer of titanium nitride (TiN) is deposited over the surface of the first titanium layer; finally a layer of TiNx is deposited over the TiN second layer. The three layers are deposited using Ion Metal Plasma (IMP) techniques which are described subsequently herein. Typically the first layer of titanium is approximately 100 xc3x85 to 200 xc3x85 thick; the second layer of TiN is about 800 xc3x85 thick, and the third layer of TiNx is about 60 xc3x85 thick. Although a good fill of contact vias having 0.25xcexc diameter through holes having an aspect ratio of about 5 was achieved, the crystal orientation of the aluminum was low in  less than 111 greater than  content, resulting in poor electromigration (EM) performance for the aluminum interconnect. Further, the reflectivity of the aluminum, measured by nanoscope, with Si as a reference, was less than about 50%, so that subsequent lithography indexing was more difficult. It is then desirable to increase the aluminum  less than 111 greater than  content for purposes of improving the EM performance and benefiting subsequent lithography process steps.
It has been discovered that an improved (Ti or TiNx)/TiN/TiNx barrier layer deposited using IMP techniques can be obtained by increasing the thickness of the first layer of Ti or TiNx to range from greater than about 100 xc3x85 to about 500 xc3x85 (the feature geometry controls the upper thickness limit); by decreasing the thickness of the TiN second layer to range from greater than about 100 xc3x85 to less than about 800 xc3x85 (preferably less than about 600 xc3x85); and, by controlling the application of the TiNx third layer to provide a Ti content ranging from about 50 atomic percent titanium (stoichiometric) to about 100 atomic percent titanium. When the first layer is TiNx, the atomic percent of Ti is at least 40 atomic percent. Preferably, the first layer is 100 atomic percent Ti. In addition, it is preferred that the TiNx third layer is formed at the end of the deposition of the TiN second layer and exhibits a Ti content gradient which begins at a stoichiometric Ti content and ends at a Ti content of about 100 atomic percent. The thickness of the TiNx third layer preferably ranges from about 15 xc3x85 to about 500 xc3x85, with the thickness of the 100 atomic percent Ti component ranging from about 15 xc3x85 to about 300 xc3x85. The improved (Ti or TiNx)/TiN/TiNx barrier layer enables the deposit of an aluminum interconnect or an aluminum via fill where the aluminum exhibits a high  less than 111 greater than  crystal orientation. Further, the aluminum layer obtained exhibits a reflectivity of 150 percent or greater at 436 nm. A (Ti or TiNx)/TiN/TiNx barrier layer having this structure, used to line a feature, enables complete filling of the feature with sputtered aluminum, where the size of the feature is about 0.25 micron and the aspect ratio is as high as about 6:1.
The improved (Ti or TiNx)/TiN/TiNx barrier layer is frequently deposited at a substrate temperature of about 200xc2x0 C. or lower; and, although the TiNx third layer may be deposited at a substrate temperature ranging from about 50xc2x0 C. to about 500xc2x0 C., it is frequently deposited at about 200xc2x0 C. or less. When the device structure is an interconnect, the TiNx third layer can be deposited at pressures ranging from about 5 mT to about 40 mT, preferably at about 25 mT. When the device feature is a via, the TiNx third layer should be deposited at reduced pressures ranging from about 5 mT to about 10 mT, preferably at about 10 mT. The aluminum fill is then deposited at a substrate temperature ranging from about 350xc2x0 C. to about 500xc2x0 C., preferably at about 400xc2x0 C. The aluminum is deposited at reduced pressures ranging from about 1 mT to about 4 mT, preferably at about 2 mT.